The unlicensed mobile access (UMA) technology has been put into use more and more widely. FIG. 1 is a system block diagram illustrating UMA technology, wherein a mobile terminal employing UMA technology may perform handover between a cellular communication network and a wireless local area network, e.g., between a GSM network and a WIFI network. In UMA technology, the choice of handover timing from a WIFI network to a GSM network is a problem.
In prior art, the most well-known mechanism of determining handover timing is a mechanism of signal strength threshold. Because of small-scale fading, etc., radio signals are sometimes not stable. Even within the coverage area of a WIFI signal, signal strength received by a mobile terminal in a short period may be low, and thus unnecessary handover will be resulted in. Therefore, the mechanism of signal strength threshold is not reliable enough.
More and more handover mechanisms based on geo-location have come into existence, which are more accurate and reliable than the mechanism of merely signal strength threshold. Some proposals suggest adopting pure WIFI solutions to solve the positioning problem of WIFI equipments. One solution is: establishing a reference point database for a plurality of reference points; storing characteristic data of the plurality of reference points, wherein the data include adjacent access points, received signal strength from a particular access point, etc.; comparing characteristic data of a WIFI equipment with the characteristic data of each reference point to find a most matched reference point and thus determine the location of the WIFI equipment; determining whether or not to perform handover according to the determined location of the WIFI equipment and other conditions, such as received signal strength.
The aforementioned solution, however, has the following disadvantages: 1) the cost is too high to establish a complete and accurate reference point database, especially in a case of a wide coverage application; 2) the characteristic data of different reference points may be so close as to result in erroneous matching and erroneous positioning.